The present invention relates to a temperature indicating composition that is UV curable, and in particular to a thermochromic, UV curable ink that can be applied to a recoverable substrate by printing.
Temperature indicating compositions have found widespread use where warning of temperature excursion is required or where a heating process is to be monitored. They are of special use where the desired maximum temperature would not otherwise be apparent to the operator; an example of this is in the use of heat-recoverable materials where a certain temperature is required for proper recovery but where a higher temperature could cause damage. Such recoverable materials are commonly used for sealing, the effectiveness of which is enhanced by provision of a heat-activatable sealant, such as a hot-melt adhesive, a reactive epoxy adhesive or a softenable mastic, on a surface of the recoverable article which will contact the substrate to be sealed. This sealant will not be visible to the installer, who therefore needs some indication that the correct bond-line temperature has been reached. The problem has been overcome by applying to a visible surface of the article to be heated a composition, the color of which changes when an internal surface, for example, has reached some desired temperature. Such temperature indication is also applicable to articles which are recovered by means other than heat, but which require heat solely for activation of an adhesive; or which may experience heat during use. Such recoverable articles may be temporarily maintained in their dimensionally unstable configuration by a hold-out member which is removed by dissolving the adhesive bond between the hold-out and the recoverable article or by other means.
A temperature indicating composition which is suitable for use on a recoverable material will contain components that are common to any paint or ink, but they will be chosen and combined with other components to insure suitability for a substrate that will be subjected to a range of temperatures, that changes size or shape, and that should remain environmentally sealing.
The basic components are a colorant, a binder or resin and often a solvent. The colorant of course gives the composition the desired color and in most paints and inks has the characteristic of permanence. The binder, together with solvent where used, wets the pigment and gives the composition tack and the rheological properties appropriate to the method by which the composition is to be applied to its substrate. Once the composition is on the substrate it is set, for example by evaporation of solvent. After application, the binder will be responsible for the final film properties of the paint or ink. In general, the colorant must function as a pigment rather than as a dye for its intended application, by which is meant that it must be opaque, rather than transparent. In order to do this it must have hiding power, and the physical properties which endows a colorant with hiding power are its refractive index and insolubility. Where the colorant alone has not a sufficiently high hiding power, a further component, called an opacifier and generally white, should be added.
The formulation of a paint or ink is related not only to the method by which it is to be applied and to the properties that the final film must possess, but also to the drying method to be used. Many methods of drying are available but only one is used in the heat-shrink art. A brief review will be made of some of the generally applicable methods available for drying inks and paints.
Drying can occur by penetration: the entire composition is absorbed into the substrate. This is of more use for dyes than pigments since a dye is entirely dissolved in its vehicle. It has not been used on heat-shrink products which are generally non-absorbent.
A second method is hardening of a binder by oxidation. This can be slow and may require catalysts to produce an acceptable rate.
The most important method of drying is by solvent evaporation, and this is the technique used for paints that have been used on recoverable products. The paint comprises a pigment suspended in a resin-solvent system, and after application the solvent is removed causing the resin to revert to a solid. This system is applicable both to paints and to the three major printing techniques: typographic, planographic, and intaglio. The rate of drying can easily be controlled: it is increased by heat, and with heat can be very rapid. Many resin-solvent systems have been developed which satisfy all the requirements as regards, odor, toxicity, color, solvent power, rate of evaporation, boiling range, combustibility and viscosity. The system is highly versatile allowing application by a variety of methods to a variety of substrates.
Polymerization of a binder can also cause the desired change from liquid to solid. A common example of this is urea formaldehyde or melamine-formaldehyde resins emulsified in a solvent such as water. Heat drives off the water, resulting in polymerization. Newer techniques involve initiating a polymerization reaction by means of infra red radiation, microwaves, dielectric, electron beams or ultra-violet radiation.
The final technique to be mentioned here is drying by means of precipitation. The vehicle in which the colorant is carried is a solvent plus binder, and a further substance is added selectively to precipitate one but not the other.
Some of the systems that have been used for temperature indicating compositions will now be reviewed.
In U.S. Pat. Nos. 4,142,416, 4,108,001 and 3,995,489 to Smith, compositions are used to detect overheating in electrical apparatus. A current of air is passed through the apparatus to cool it, and parts of the apparatus that are cooled by the current of air are coated with an organic composition which decomposes and thermoparticulates. If overheating occurs, particles of the decomposed composition will be detected downstream of the apparatus. The specification is not concerned with themochromism, but it mentions that the organic compounds used char when heated.
U.S. Pat. No. 3,816,335 discloses incorporation of an inorganic thermochromic colorant throughout the body of a heat-recoverable plastic article. A problem that can occur with some inorganic systems is a tendency for the base polymer to become degraded under the action of the colorant and heat. This problem was solved in U.S. Pat. No. 4,105,583 where it was proposed that zinc borate and zinc sulfide should be added to some inorganic systems. Other active ingredients were also mentioned. The disclosures of this patent and of U.S. Pat. No. 3,816,335 are incorporated herein by reference. In U.S. Pat. No. 4,344,909, a move was made away from inorganic colorants to the use of organic materials that melt and decompose at the desired temperature, thus causing a color change. In this case the thermochromic ingredient is applied in an aqueous or other vehicle, or as part of a solid formulation such as a wax. The disclosure of this publication is incorporated herein by reference.
This use of organic materials on recoverable articles can be improved by selection of the binder so that decomposition products of the thermochromic colorant were trapped thereby insuring irreversibility. This is disclosed in U.S. Pat. No. 4,450,023, the disclosure of which is incorporated herein by reference.
The majority of these compositions comprise a pigment, an opacifier, a binder and a solvent. They thus rely, like many paints and inks, on evaporation of a solvent. These compositions have no toxicity problem and they can provide a sharp color change at the desired temperature. Under certain rather exacting circumstances, however, they can have some drawbacks. In particular, solvent removal requires energy, and if the solvent evaporated is not re-circulated, a further expense is incurred. Drying times, although short, are often rate-limiting steps in the manufacture of recoverable coated material. A solvent system often results in a thick, sometimes rather dusty, coating of the temperature indicating composition. When a heat-shrink article is recovered by torch any thick areas of coating can concentrate the heat causing hot-spots and cratering of the surface of the article. This is likely to be a problem only with unskilled hands in unfavorable conditions where a hot flame is required; nonetheless it could be regarded as an area of craft-sensitivity which is desirable to reduce.
What we have now designed is a temperature indicating composition which can be applied to a recoverable substrate extremely rapidly by printing to produce a very thin coating having sufficient hiding power which will maintain its adhesion during the conformational change that occurs on recovery.